Method of producing a steel product having an oxidation-resistant coating

ABSTRACT

A coating of oxidation-resistant material is deposited on a mild steel substrate, the coating having carbide-forming constituents. The coated substrate is subjected to heat treatment causing diffusion, the heat treatment comprising three sequential stages: a preliminary diffusion stage in a dry hydrogen atmosphere, a decarburization stage in a decarburizing atmosphere, and a final diffusion stage in a dry hydrogen atmosphere. The resulting product has a decarburized oxidation-resistant coating.

United States Patent [1 1 Leroy et al.

[ Feb. 25, 1975 METHOD OF PRODUCING A STEEL PRODUCT HAVING AN OXIDATION-RESISTANT COATING [75] Inventors: Vincent Marguerite Leroy, Liege;

Roland Cesar Amand Liesenborghs, Sclessin, both of Belgium [73] Assignee: Societe Anonyme Cockerill-Ougree-Providence et esperance-Longdoz en Abrege Cockerill, Seraing, Belgium [22] Filed: Jan. 30, 1974 [21] Appl. No.: 437,916

[30] Foreign Application Priority Data Jan. 30, 1973 Belgium 79475 [52] US. Cl 148/12.l,117/22,1l7/105.2, 148/6, 148/16, 148/127 [51] Int. Cl. C23c 13/02 [58] Field of Search ..148/16,127, 6, 12.1, 31.5; 117/22, 105.2

[56] References Cited UNITED STATES PATENTS 2,536,774 l/1951 Samuel 148/6 2,899,332 8/1959 Samuel 117/22 3,340,054 9/1967 Ward et a1. 117/22 3,589,927 6/1971 Holker et al 117/22 3,623,901 11/1971 Forstmann et al 117/22 Primary ExaminerW. Stallard Attorney, Agent, or FirmHolman & Stern [57] ABSTRACT 8 Claims, No Drawings METHOD OF PRODUCING A STEEL PRODUCT HAVING AN OXIDATION-RESISTANT COATING The present invention relates to a method of forming an oxidation-resistant coating on a steel substrate, such as steel sheet or wire, this coating comprising an oxidation-resistant material consisting of one or more of the metals Cr, Ni, Co, M0, or an alloy of two or more of these metals with one another or an alloy of one or more of these metals with iron or with other elements, the coating being formed by depositing the oxidationresistant material on the steel substrate (eg. by metalization or powder compacting techniques) and then subjecting the coated substrate to heat treatment to cause diffusion of the constituents of the substrate and coating.

The most important use of oxidation-resistant coatings is in the protection of ferrous products from corrosion and oxidation, and to this end it has already been suggested to employ powders comprising one or more of the metals Cr, Ni, Co and M or their alloys, whose protective properties are well known. With steel substrates it has in the past been found that satisfactory results, as far as corrosion resistance in an aqueous medium is concerned, could be obtained in practice with titanium-stabilized steels only.

Methods used at present include carefully cleaning the substrate, then depositing an alloy on the substrate by spray metallization or by spreading and compacting of powder, and finally heat treating the coated substrate at a high temperature in a dry hydrogen atmosphere in order to cause diffusion of the alloy, deoxidation of the coating, and also decarburization of the substrate and the coating. After this, the coated product can be rolled and recrystallization annealing can take place depending upon the mechanical and structural properties desired in the final product. Tests carried out on these coated products have shown that the quality of the corrosion resistance of these coatings is due to the absence of any intergranual carbides of the oxidation-resistant elements of the coating; this occurs with titanium stabilized steel substrates because the formation of titanium carbides means that only a very small amount of free carbon, i.e.. carbon not combined with the titanium is present.

When such a process is used with mild steels other than titanium-stabilized ones. it is found that when tested for corrosion in an aqueous medium. the coating exhibits pitting due to the presence of carbides of the oxidation-resistant elements. This phenomenon is explained as follows: before deposition of the coating, carbon in the steel substrate is not fixed in the form of titanium carbides, so this un-fixed carbon, also called free carbon, is then available for forming carbides with the oxidation-resistant elements of the coating during diffusion heat treatment and during subsequent cooling, this resulting in a reduction in the corrosion resis tance of the coating and in the formation of pits.

What is desired is a method of forming an oxidationresistant coating which does not undergo corrosion in an aqueous medium even when the substrate consists of ordinary steel, such as rimming steel or aluminum killed steel, whose carbon content is at least about 0.050 percent (by weight), this carbon being free."

In accordance with the present invention, the heat treatment comprises the following three stages in sequcnce:

a. a preliminary diffusion stage in a dry hydrogen atmosphere;

b. a decarburization stage in a decarburizing atmosphere, such as carbon dioxide atmosphere or a wet hydrogen atmosphere; and

c. a final diffusion stage in a dry hydrogen atmo sphere.

The method is thus characterized in that diffusion takes place in an active atmosphere whose composition varies in the course of time. The temperature may also vary from stage to stage.

In order to improve the efficiency of the exchange reactions between the active atmosphere and the coated substrate, it is preferred to provide a gap or space between adjacent faces or surfaces in the case of flat products. When dealing with wire coils, these optimum conditions are met automatically.

It should be noted that the three stages of the heat treatment concerned can be carried out during a single annealing cycle.

The preliminary diffusion stage is intended to ensure adhesion of the coating to the substrate and simultaneous removal of the oxygen present in the coating, and a dry hydrogen atmosphere is necessary in order to avoid oxidation of the interface between substrate and coating either due to the presence of this oxygen (the oxygen is eliminated as H O) or due to the action of that oxygen which would be brought about by an active gas whose dew point is not sufficiently low. Such oxidation of the interface would adversely affect diffusion and would inhibit adhesion of the coating. The decarburization stage is preferably carried out under a wet hydrogen atmosphere for well-known reasons of efficiency. The final diffusion stage is designed to complete the diffusion of the constituents of the coating and to prevent excessive oxidation of the surface of the substrate as a consequence of the preceding stage carried out in a wet hydrogen atmosphere.

It has been found advantageous for the three stages of the heat treatment to be carried out under the following conditions of temperature and duration:

a. the preliminary diffusion stage at a temperature of 700C to l300C (preferably l000C to l250C) for 16 hours to A hour;

b. the decarburization stage at a temperature of 900C to l300C for 4 hours to A hour according to the thickness of the coated substrate; and

c. the final diffusion stage at a temperature of lO00C to l300C for 48 to 10 hours.

The coating material may be deposited as a powder of one or more of the metals Cr, Ni, Co M0, or alloys of these metals, preferably an iron alloy of Cr or of Ni- Cr. The carbon contentof the powder will be between 0.020 and 1 percent.

The deposition of the coating material is advantageously carried out either by metalization by means of a flame torch or an arc torch, or by a distribution or spreading operation followed by a compacting operation.

It is well known that these deposition techniques produce coatings whose thickness is of the order of 50 to I00 micrometres.

Among the coated products obtained by the above described method, the present invention particularly aims to a product whose coating advantageously has a carbon content lower than 0.020 percent.

a The coated substrate obtained may The above-described method enables a coating to be obtained whose microstructure is free from any intergranularcarbide, this ensuring good corrosion resistance in an aqueous medium.

be subjected to a reducing operation (such as rolling), followed by recrystallization annealing, if desired. Furthermore the coating obtained provides an improvement (with respect to other processes) in the surface finish after rolling and recrystallization.

EXAMPLE A coating material consisting of the alloy Fe-75 Cr containing 0.040 wt. percent carbon was disposited on a mild steel substrate having a carbon content of 0.050 wt. percent. The thickness of the deposit was approximately 100 micrometres. (According to the conventional technique a heat treatment consisting of diffusion for 16 hours at 1 150C in dry hydrogen leads to the formation of a coating having a carbon content between 0.150 percent and 0.200 percent by weight). The best treatment consisted of the following three stages, preliminary diffusion for half an hour at 1 150C in a dry hydrogen atmosphere, decarburization at 900C for 2 hours in a wet hydrogen atmosphere; and final diffusion for 15 /2 hours at 1 150C in a dry hydrogen atmosphere. This lead to the formation of an Fe-75Cr coating having a carbon content of 0.010 to 0.0l5 percent by weight.

We claim:

1. A method of producing a steel product having a decarburized oxidation-resistant coating, comprising the steps of depositing on a mild steel substrate a coating of at least one oxidation-resistant material selected from the group consisting of the metals Cr, Ni, Co, and Mo, alloys of at least two of these metals with each other, and alloys of at least one of these metals with iron or other elements; and then subjecting the coated substrate to heat treatment to cause diffusion of constituents of the substrate and coating, the heat treatment comprising the following three stages in sequence;

a. a preliminary diffusion stage in a dry hydrogen atmosphere;

b. a decarburization stage in a decarburizing atmosphere; and

c. a final diffusion stage in a dry hydrogen atmosphere.

2. A method as claimed in claim 1, in which the decarburizing atmosphere is a wet hydrogen atmosphere.

3. A method as claimed in claim 1, in which the three stages of the heat treatment are carried out under the following conditions of temperature and duration:

21. the preliminary diffusion stage at a temperature of 700C to 1300C for l6 hours to A hour;

b. the decarburization stage at a temperature from 900C to 1300C for 4 hours to A hour; and

c. the final diffusion stage at a temperature of 1000C to l300C for 48 to 10 hours.

4. A method as claimed in claim 1, in which the oxidation-resistant material is deposited as a powder.

5. A method as claimed in claim 4, in which a powder of iron-chromium alloy is deposited on the substrate.

6. A method as claimed in claim-4, in which a powder of an iron-alloy of nickel and chromium is deposited on the substrate.

7. A method as claimed in claim 1, further comprising subjecting the coated substrate to a reducing operation.

8. A method as claimed in claim 7, further comprising subjecting the reduced coated substrate to a recrystallization annealing operation. 

1. A METHOD OF PRODUCING A STEEL PRODUCT HAVING A DECARBURIZED OXIDATION-RESISTANT COATING, COMPRISING THE STEPS OF DEPOSITING ON A MILD STEEL SUBSTRATE A COATING OF AT LEAST ONE OXIDATION-RESISTANT MATERIAL SELECTED FROM THE GROUP CONSISTING OF THE METALS CR, NI, CO, AND MO, ALLOYS OF AT LEAST TWO OF THESE METALS WITH EACH OTHER, AND ALLOYS OF AT LEAST ONE OF THESE METALS WITH IRON OR OTHER ELEMENTS; AND THEN SUBJECTING THE COATED SUBSTRATE TO HEAT TREATMENT TO CAUSE DIFFUSION OF CONSTITUENTS OF THE SUBSTRATE AND COATING, THE HEAT TREATMENT COMPRISING THE FOLLOWING THREE STAGES IN SEQUENCE; A. A PRELIMINARY DIFFUSION STAGE IN A DRY HYDROGEN ATMOSPHERE; B. A DECARBURIZATION STAGE IN A DECARBURIZING ATMOSPHERE; AND C. A FINAL DIFFUSION STAGE IN A DRY HYDROGEN ATMOSPHERE.
 2. A method as claimed in claim 1, in which the decarburizing atmosphere is a wet hydrogen atmosphere.
 3. A method as claimed in claim 1, in which the three stages of the heat treatment are carried out under the following conditions of temperature and duration: a. the preliminary diffusion stage at a temperature of 700*C to 1300*C for 16 hours to 1/4 hour; b. the decarburization stage at a temperature from 900*C to 1300*C for 4 hours to 1/4 hour; and c. the final diffusion stage at a temperature of 1000*C to 1300*C for 48 to 10 hours.
 4. A method as claimed in claim 1, in which the oxidation-resistant material is deposited as a powder.
 5. A method as claimed in claim 4, in which a powder of iron-chromium alloy is deposited on the substrate.
 6. A method as claimed in claim 4, in which a powder of an iron-alloy of nickel and chromium is deposited on the substrate.
 7. A method as claimed in claim 1, further comprising subjecting the coated substrate to a reducing operation.
 8. A method as claimed in claim 7, further comprising subjecting the reduced coated substrate to a recrystallization annealing operation. 